Sliced routing table management with replication

ABSTRACT

Techniques are provided for hash-based routing table management in a distributed network switch. A frame having a source address and a destination address is received by a switch module having bridge elements and a routing table divided into slices of buckets, each slice having a respective property and including one or more buckets. If a routing entry for the source address is found in a first slice of a first set of buckets of the routing table responsive to a lookup request for the source address, and the property of the first slice satisfies a replication condition, then the routing entry is replicated to a second set of buckets of the routing table.

BACKGROUND

Computer systems often use multiple computers that are coupled together in a common chassis. The computers may be separate servers that are coupled by a common backbone within the chassis. Each server is a pluggable board that includes at least one processor, an on-board memory, and an Input/Output (I/O) interface. Further, the servers may be connected to a switch to expand the capabilities of the servers. For example, the switch may permit the servers to access additional Ethernet networks or Peripheral Component Interconnect Express (PCIe) slots as well as permit communication between servers in the same or different chassis. In addition, multiple switches may also be combined to create a distributed network switch.

SUMMARY

Embodiments of the invention provide a method, computer-readable medium, and system for performing an operation for hash-based routing table management in a distributed network switch including a set of switch modules. The switch modules include at least a first switch module. The operation includes receiving, by the first switch module, a first frame having a source address and a destination address. The first switch module includes bridge elements and a routing table. The routing table in the first switch module is shared among the bridge elements in the first switch module and includes sets of buckets. Each set of buckets is associated with a respective hash function of a set of hash functions and is divided into slices of buckets. Each slice has a respective property and includes one or more buckets. The operation further includes, upon accessing a routing entry for the source address in a first slice of a first set of buckets of the routing table responsive to a first subsequent lookup request for the source address, and determining that the property of the first slice satisfies a replication condition, replicating the accessed routing entry to at least a second set of buckets of the routing table.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited aspects are attained and can be understood in detail, a more particular description of embodiments of the invention, briefly summarized above, may be had by reference to the appended drawings.

It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

FIG. 1 illustrates a system architecture that includes a distributed network switch, according to one embodiment of the invention.

FIG. 2 illustrates the hardware representation of a system that implements a distributed network switch, according to one embodiment of the invention.

FIG. 3 illustrates a distributed network switch, according to one embodiment of the invention.

FIG. 4 is a block diagram depicting components of a switch module of the distributed network switch, according to one embodiment of the invention.

FIG. 5 is a block diagram depicting components of a routing table of the switch module, according to one embodiment of the invention.

FIG. 6 is a flowchart depicting a method for hash-based routing table management in the distributed network switch, according to one embodiment of the invention.

FIG. 7 is a flowchart depicting a method to modify the routing table based on distinct hash functions, according to one embodiment of the invention.

FIG. 8 is a flowchart depicting a method to update the routing table to reflect a source address, according to one embodiment of the invention.

FIG. 9 is a flowchart depicting a method to insert a routing entry into the routing table, according to one embodiment of the invention.

FIG. 10 is a flowchart depicting a method for sliced routing table management in the distributed network switch, according to one embodiment of the invention.

FIG. 11 is a data flow diagram illustrating operations for managing a cached routing table, according to one embodiment of the invention.

FIG. 12 is a flowchart depicting a method for cached routing table management in the distributed network switch, according to one embodiment of the invention.

FIG. 13 is a flowchart depicting a method for accessing a cached routing table in the distributed network switch, according to one embodiment of the invention.

FIG. 14 is a flowchart depicting a method for determining to invalidate a lookup cache, according to one embodiment of the invention.

FIG. 15 is a block diagram depicting components for sliced routing table management with replication, according to one embodiment of the invention.

FIG. 16 is a flowchart depicting a method for sliced routing table management with replication, according to one embodiment of the invention.

FIG. 17 is a flowchart depicting a method for determining whether to replicate a routing entry in the distributed network switch, according to one embodiment of the invention.

FIG. 18 is a flowchart depicting a method for inserting a new, non-replicated routing entry based on a priority policy, according to one embodiment of the invention.

FIG. 19 is a flowchart depicting a method for applying an aging insertion policy, according to one embodiment of the invention.

DETAILED DESCRIPTION

Embodiments of the invention provide techniques for hash-based routing table management in a distributed network switch. One embodiment provides a first switch module configured to receive a first frame having a source address and a destination address. Upon determining that a routing entry for the source address does not exist in a routing table of the first switch module, the first switch module determines routing information for the source address and generates the routing entry for the source address based on the routing information. The first switch module then modifies the routing table to include the routing entry generated for the source address and based on a plurality of distinct hash functions. Using the techniques for hash-based routing table management disclosed herein may facilitate quicker access to the routing table at least in some cases, relative to alternative approaches for managing the routing table. Accordingly, the distributed network switch may operate more efficiently at least in some cases.

In the following, reference is made to embodiments of the invention. However, it should be understood that the invention is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice the invention. Furthermore, although embodiments of the invention may achieve advantages over other possible solutions and/or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the invention. Thus, the following aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). Likewise, reference to “the invention” shall not be construed as a generalization of any inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s).

As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer readable storage medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of this document, a computer readable storage medium may be any tangible medium that can contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device.

A computer readable signal medium may include a propagated data signal with computer readable program code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electro-magnetic, optical, or any suitable combination thereof. A computer readable signal medium may be any computer readable medium that is not a computer readable storage medium and that can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc., or any suitable combination of the foregoing.

Computer program code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The program code may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider).

Aspects of the present invention are described below with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks.

The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.

FIG. 1 illustrates a system architecture that includes a distributed network switch, according to one embodiment of the invention. The first server 105 may include at least one processor 109 coupled to a memory 110. The processor 109 may represent one or more processors (e.g., microprocessors) or multi-core processors. The memory 110 may represent random access memory (RAM) devices comprising the main storage of the server 105, as well as supplemental levels of memory, e.g., cache memories, non-volatile or backup memories (e.g., programmable or flash memories), read-only memories, and the like. In addition, the memory 110 may include memory storage physically located in the server 105 or on another computing device coupled to the server 105.

The server 105 may operate under the control of an operating system 107 and execute various computer software applications, components, programs, objects, modules, and data structures, such as virtual machines 111.

The server 105 may include network adapters 115 (e.g., converged network adapters, or CNAs). A converged network adapter may include single root I/O virtualization (SR-IOV) adapters such as a Peripheral Component Interconnect Express (PCIe) adapter that supports Converged Enhanced Ethernet (CEE). Another embodiment of the system 100 may include a multi-root I/O virtualization (MR-IOV) adapter. The network adapters 115 may further be used to implement a Fiber Channel over Ethernet (FCoE) protocol, RDMA over Ethernet, Internet small computer system interface (iSCSI), and the like. In general, a network adapter 115 transfers data using both an Ethernet and PCI based communication method and may be coupled to one or more of the virtual machines 111. In particular, Ethernet may be used as the protocol to the switch fabric, while PCI may be used as the protocol to transfer data to/from main memory to the network adapter 115. Additionally, the adapters may facilitate shared access between the virtual machines 111. While the adapters 115 are shown as being included within the server 105, in other embodiments, the adapters may be physically distinct devices that are separate from the server 105.

In one embodiment, each network adapter 115 may include a converged adapter virtual bridge (not shown) that facilitates data transfer between the adapters 115 by coordinating access to the virtual machines 111. Each converged adapter virtual bridge may recognize data flowing within its domain (i.e., addressable space). A recognized domain address may be routed directly without transmitting the data outside of the domain of the particular converged adapter virtual bridge.

Each network adapter 115 may include one or more Ethernet ports that couple to one of the bridge elements 120, also referred to herein as bridging elements. Additionally, to facilitate PCIe communication, the server may have a PCI Host Bridge 117. The PCI Host Bridge would then connect to an upstream PCI port 122 on a switch element in the distributed network switch 180. The data is then routed via the switching layer 130 to the correct downstream PCI port 123 which may be located on the same or different switch module as the upstream PCI port 122. The data may then be forwarded to the PCI device 150.

The bridge elements 120 may be configured to forward data frames throughout the distributed network switch 180. For example, a network adapter 115 and bridge element 120 may be connected using two 40 Gbit Ethernet connections or one 100 Gbit Ethernet connection. The bridge elements 120 forward the data frames transmitted by the network adapter 115 to the switching layer 130. The bridge elements 120 may include a lookup table that stores address data used to forward the received data frames. For example, the bridge elements 120 may compare address data associated with a received data frame to the address data stored within the lookup table. Thus, the network adapters 115 do not need to know the network topology of the distributed network switch 180.

The distributed network switch 180, in general, includes a plurality of bridge elements 120 that may be located on a plurality of a separate, though interconnected, hardware components. To the perspective of the network adapters 115, the distributed network switch 180 acts like one single switch even though the distributed network switch 180 may be composed of multiple switches that are physically located on different components. Distributing the network switch 180 provides redundancy in case of failure.

Each of the bridge elements 120 may be connected to one or more transport layer modules 125 that translate received data frames to the protocol used by the switching layer 130. For example, the transport layer modules 125 may translate data received using either an Ethernet or PCI communication method to a generic data type (i.e., a cell) that is transmitted via the switching layer 130 (i.e., a cell fabric). Thus, the switch modules comprising the distributed network switch 180 are compatible with at least two different communication protocols—e.g., the Ethernet and PCIe communication standards. That is, at least one switch module has the necessary logic to transfer different types of data on the same switching layer 130.

Although not shown in FIG. 1, in one embodiment, the switching layer 130 may comprise a local rack interconnect (LRI) which connects bridge elements 120 located within the same chassis and rack, as well as links that connect to bridge elements 120 in other chassis and racks.

After routing the cells, the switching layer 130 may communicate with transport layer modules 126 that translate the cells back to data frames that correspond to their respective communication protocols. A portion of the bridge elements 120 may facilitate communication with an Ethernet network 155 which provides access to a LAN or WAN (e.g., the Internet). Moreover, PCI data may be routed to a downstream PCI port 123 that connects to a PCIe device 150. The PCIe device 150 may be a passive backplane interconnect, as an expansion card interface for add-in boards, or common storage that can be accessed by any of the servers connected to the distributed network switch 180.

A second server 106 may include a processor 109 connected to an operating system 107 and memory 110 which includes one or more virtual machines 111 similar to those found in the first server 105. The memory 110 of server 106 also includes a hypervisor 113 with a virtual bridge 114. The hypervisor 113 manages data shared between different virtual machines 111. Specifically, the virtual bridge 114 allows direct communication between connected virtual machines 111 rather than requiring the virtual machines 111 to use the bridge elements 120 or switching layer 130 to transmit data to other virtual machines 111 communicatively coupled to the hypervisor 113.

An Input/Output Management Controller (IOMC) 140 (i.e., a special purpose processor) is coupled to at least one bridge element 120 which provides the IOMC 140 with access to the switching layer 130. One function of the IOMC 140 may be to receive commands from an administrator to configure the different hardware elements of the distributed network switch 180. In one embodiment, these commands may be received from a separate switching network from the switching layer 130.

Although one IOMC 140 is shown, the system 100 may include a plurality of IOMCs 140. In one embodiment, these IOMCs 140 may be arranged in a hierarchy such that one IOMC 140 is chosen as a master while the others are delegated as members.

FIG. 2 illustrates a hardware level diagram of the system 100, according to one embodiment of the invention. Server 210 and 212 may be physically located in the same chassis 205; however, the chassis 205 may include any number of servers. The chassis 205 also includes a plurality of switch modules 250, 251 that include one or more sub-switches 254. In one embodiment, the switch modules 250, 251, 252 are hardware components (e.g., PCB boards, FPGA boards, system on a chip, etc.) that provide physical support and connectivity between the network adapters 115 and the bridge elements 120. In general, the switch modules 250, 251, 252 include hardware that connects different chassis 205, 207 and servers 210, 212, 214 in the system 200.

The switch modules 250, 251, 252 (i.e., a chassis interconnect element) include one or more sub-switches 254 and an IOMC 255, 256, 257. The sub-switches 254 may include a logical or physical grouping of bridge elements 120. Each bridge element 120 may be physically connected to the servers 210, 212. For example, a bridge element 120 may route data sent using either Ethernet or PCI communication protocols to other bridge elements 120 attached to the switching layer 130. However, in one embodiment, the bridge element 120 may not be needed to provide connectivity from the network adapter 115 to the switching layer 130 for PCI or PCIe communications.

Each switch module 250, 251, 252 includes an IOMC 255, 256, 257 for managing and configuring the different hardware resources in the system 200. In one embodiment, the respective IOMC for each switch module 250, 251, 252 may be responsible for configuring the hardware resources on the particular switch module. However, because the switch modules are interconnected using the switching layer 130, an IOMC on one switch module may manage hardware resources on a different switch module.

The dotted line in chassis 205 defines the midplane 220 between the servers 210, 212 and the switch modules 250, 251. That is, the midplane 220 includes the data paths that transmit data between the network adapters 115 and the sub-switches 254.

Each bridge element 120 connects to the switching layer 130. In addition, a bridging element 120 may also connect to a network adapter 115 or an uplink. As used herein, an uplink port of a bridging element 120 provides a service that expands the connectivity or capabilities of the system 200. As shown in chassis 207, one bridging element 120 includes a connection to an Ethernet or PCI connector 260. For Ethernet communication, the connector 260 may provide the system 200 with access to a LAN or WAN (e.g., the Internet). Alternatively, the port connector 260 may connect the system to a PCIe expansion slot—e.g., PCIe device 150. The device 150 may be additional storage or memory which each server 210, 212, 214 may access via the switching layer 130. Advantageously, the system 200 provides access to a switching layer 130 that has network devices that are compatible with at least two different communication methods.

As shown, a server 210, 212, 214 may have a plurality of network adapters 115. This provides redundancy if one of these adapters 115 fails. Additionally, each adapter 115 may be attached via the midplane 220 to a different switch module 250, 251, 252. As illustrated, one adapter of server 210 is communicatively coupled to a bridge element 120 located in switch module 250 while the other adapter is connected to a bridge element 120 in switch module 251. If one of the switch modules 250, 251 fails, the server 210 is still able to access the switching layer 130 via the other switching module. The failed switch module may then be replaced (e.g., hot-swapped) which causes the IOMCs 255, 256, 257 and bridging elements 120 to update the routing tables and lookup tables to include the hardware elements on the new switching module.

FIG. 3 illustrates a virtual switching layer, according to one embodiment of the invention. Each bridging element 120 in the systems 100 and 200 are connected to each other using the switching layer 130 via a mesh connection schema. That is, no matter the bridging element 120 used, a cell (i.e., data packet) can be routed to another bridging element 120 located on any other switch module 250, 251, 252. This may be accomplished by directly connecting each bridging element 120—i.e., each bridging element 120 has a dedicated data path to every other bridging element 120. Alternatively, the switching layer 130 may use a spine-leaf architecture where each bridging element 120 (i.e., a leaf node) is attached to at least one spine node. The spine nodes route cells received from the bridging elements 120 to the correct spine node which then forwards the data to the correct bridging element 120. However, this invention is not limited to any particular technique for interconnecting the bridging elements 120.

Accordingly, the distributed network switch disclosed herein is configured to provide Layer 2 Ethernet switching via: multiple switch modules 250, 251, the LRI interconnecting the switch modules 250, 251, and management firmware executing on a management controller such as the IOMC 255, 256, 257. Doing so may eliminate the need for stand-alone Ethernet switches to interconnect processing elements in the servers 105, 106. At least in some embodiments, the networked system may also be augmented in a granular and/or scalable manner by adding individual components such as switch modules and/or processing elements.

In one embodiment, each switch module 250, 251 is operatively connected to multiple bridge elements. In a particular embodiment, each switch module is configured to use five bridge elements, where each bridge element is a distributed Virtual Ethernet Bridge (dVEB) that supports data transfer rates of one hundred gigabits per second (100 Gbps). The dVEBs are managed by firmware executing on the management controller.

FIG. 4 is a block diagram depicting components of the switch module 250 of FIG. 2, according to one embodiment of the invention. As shown, the switch module 250 includes multiple bridge elements 402 ₁₋₅ and a routing table 404 shared between the multiple bridge elements, also referred to as a forwarding data base (FDB). In this particular example, the bridge elements 402 ₁₋₅ are dVEBs. Further, as described above, each switch module is operatively connected to an LRI 406. Each bridge element 402 ₁₋₅ is configured to send a routing information lookup request to the routing table 404, and routing table 404 is configured to send a routing information lookup response to the requesting bridge element.

In one embodiment, responsive to an Ethernet frame entering an ingress port, the switch module accesses the routing table to determine if the source and/or destination addresses are present in the routing table. For example, each bridge element 402 ₁₋₅ generates a destination address lookup and a source address lookup per frame to the routing table. Each bridge element 402 ₁₋₅ then uses routing information received from the lookups to send the frame to the destination. At least in some embodiments, it is desirable that such lookups have direct access to a complete set of learned routing information in the distributed network switch. If only a subset of the routing information is cached locally, such as is further described below, the switch module may need to query a non-local routing table, introducing additional latency and/or complexity to the lookups. On the other hand, the complete set of learned routing information quickly become large when virtualization is used.

For example, a networked system having 640 server nodes each configured to support 256 MAC addresses results in 163,840 possible MAC addresses that need to be accounted for within the networked system. To facilitate managing routing tables of sufficient size to accommodate such networked systems, the routing tables may be shared among bridge elements as described herein—as opposed to each bridge element having its own respective routing table. Any routing table indexing scheme used should take into account the size of the MAC addresses, e.g., 48 bits in length, and any property of the MAC addresses, such as the property of having no inherent ordering. Further, as Ethernet link speeds increase, such as to 100 Gbit/s and beyond, in order to keep up with 64-byte Ethernet frames, a frame needs to be handled every 6.72 nanoseconds on each port, including two address lookups per frame. Additional routing table processing bandwidth per frame is also desirable to perform other operations such as management and refresh operations. At least in some embodiments, lookup responses are received by a switch module in the same order as the lookup requests were previously sent by the switch module.

FIG. 5 is a block diagram depicting components of the routing table 404 of FIG. 4, according to one embodiment of the invention. The routing table 404 includes a predetermined number of sets of buckets, also referred to herein as groups of buckets. In one embodiment, each set is implemented as a hash table 502 ₁₋₄, each hash table 502 ₁₋₄ having a predetermined number of buckets, such as bucket 504, and each bucket being configured to store up to a predetermined number of routing entries configured to store routing information. In some embodiments, the size of the routing table 404 is predetermined to yield a routing table storage utilization of 50% or lower, based on a current number of routing entries expected. Further, each hash table may include a predetermined number of slices 506 of buckets, also referred to herein as subgroups of buckets or subsets of buckets. In one embodiment, each slice is implemented as an independent array data structure configured to support parallel access, relative to the other slices. For instance, a configuration of four hash tables, each having four slices, provides up to sixteen parallel accesses to the routing table per clock cycle. Although the configuration of four slices per hash table has shown to be useful at least in some cases, other counts of slices per hash table may be used without departing from the scope of the present disclosure. Each slice may be associated with one or more properties specific to the respective slice. For example, each slice may be associated with an access frequency property characterizing the respective slice. Access to the slices may be managed by a predetermined entity, such as an arbiter component of the routing table. In some alternative embodiments, rather than being divided into slices, each hash table itself is implemented as a single, independent array data structure.

In one embodiment, each hash table is identifiable via a hash table identifier that is distinct within the routing table. Further, each bucket is identifiable via a bucket identifier that is distinct within the hash table of the respective bucket. Further still, each slice is identifiable via a slice identifier that is distinct within the hash table of the respective slice. Depending on the embodiment, the slice identifier may be a subset of bits of the bucket identifier or may be separate from the bucket identifier. Similarly, the hash table may be a subset of bits of the bucket identifier (and/or of the slice identifier) or may be separate from the bucket identifier and/or slice identifier.

In one embodiment, each hash table 502 ₁₋₄ is associated with a distinct hash function. At least in some embodiments, the distinct hash functions are used to reduce a frequency of collisions of routing table entries. To this end, one or more of the hash functions may be chosen based on a determined or estimated collision property representing a frequency of collisions of routing table entries for a predefined set of distinct inputs. The collision property may also be referred to herein as an intra-hash collision property. Further, one or more of the hash functions may be chosen based on a determined or estimated avalanche property of the respective hash function. A hash function is regarded as having a strong avalanche property if a small change to the input to the hash function results in a large change in the output. For example, a hash function is said to have a strong avalanche property if a change in a single bit of the input results in many bits being changed in the output.

Additionally or alternatively, in one embodiment, each hash function may be chosen based on a determined or estimated inter-hash collision property such that any two distinct inputs yielding a collision in a given hash table should not yield a collision in any of the other hash tables—or should collide in as few of the other hash tables as possible. In other words, any two distinct inputs hashing to a single bucket in one hash table should not hash to a single bucket in any of the other hash tables. At least in some embodiments, the distinct hash functions are chosen to minimize the number of two distinct inputs hashing to a single bucket in multiple hash tables. Further, each hash function may be chosen based on a predefined property of the slices. The property may include any data characterizing each slice relative to the other slices. For example, each hash function may be chosen based on a determined or estimated inter-slice distribution property such that incoming address lookup requests result in a uniform distribution of accesses across the slices of each hash table.

In one embodiment, each of the avalanche property, the intra-hash collision property, the inter-hash collision property, and the inter-slice distribution property may be determined or estimated by applying the hash function to a predefined set of inputs or based on one or more predefined property estimation rules provided by a user. At least in some embodiments, these properties of hash functions may also be evaluated during runtime by the arbiter component of the routing table. In one embodiment, the number of access requests to the slices may be as large as a count of bridge elements per switch module multiplied by a count of lookups per bridge element further multiplied by a count of hash table requests per lookup, e.g., 5*2*4=40. In one embodiment, assuming each frame is sixty-four bytes in size and spans a window of three clock cycles, if the routing table includes sixteen total slices across all hash tables, then a maximum of forty-eight accesses may be accommodated over the window of three clock cycles. The maximum is sufficient to service even a worst-case scenario in which all ports are simultaneously receiving back-to-back, sixty-four-byte frames and in which forty accesses are requested per three-cycle window.

In one embodiment, the routing table is configured to support a normal insertion mode and a bandwidth insertion mode. The bandwidth insertion mode is also referred to herein as an access frequency mode or a bandwidth-balancing mode. At least in some embodiments, a user may specify which mode is the active mode. The arbiter component of the routing table may monitor, for each slice in the routing table, a rate of successful routing entry lookups in the respective slice. The rate is also referred to herein as a hit rate. When the bandwidth insertion mode is active, the arbiter component determines a set of candidate slices from different hash tables, into which to insert a new routing entry. The arbiter component then selects the candidate slice having a lowest hit rate, as the slice into which to insert the new routing entry. In contrast, when in normal insertion mode, the selection of a hash table and/or a slice into which to insert a new routing entry is not dependent on the monitored hit rates. Normal insertion mode is further described below in conjunction with FIGS. 6-8. Bandwidth insertion mode is further described below in conjunction with FIG. 9.

In one embodiment, each hash function may be cryptographic or non-cryptographic. Examples of non-cryptographic hash functions include, without limitation, Pearson hash function, Fowler-Noll-Vo hash function, Zobrist hash function, Jenkins hash function, Bernstein hash function, elf64 hash function, MurmurHash function, SpookyHash function, and CityHash function. Examples of cryptographic hash functions include, without limitation, Gosudarstvennyi Standard Soyuza SSR (GOST) hash function, Hash of Variable Length (HAVAL) hash function, message digest algorithm hash function (such as MD2, MD4, and MD5), PANAMA hash function, RadioGatún hash function, RACE Integrity Primitives Evaluation Message Digest (RIPEMD) hash function, secure hash algorithm (SHA) hash function, Tiger hash function, and Whirlpool hash function.

In one embodiment, a routing entry is inserted, updated, and/or retrieved into a given hash table, by using the hash function associated with the given hash table to generate a hash value based on a routing key of the routing entry. The hash value may then be used as a bucket identifier to identify a bucket of the hash table associated with the hash function used. As described above, in embodiments where the hash tables are divided into slices, a predetermined number of bits of the hash value may be used to identify a particular slice. For example, two bits of the hash value may be used to uniquely identify one of the four slices of a hash table. In some embodiments, a hash value from each hash function is generated using the routing key, and the identified buckets, also referred to herein as candidate buckets, are evaluated to determine a least-full bucket (i.e., among the candidate buckets), into which the routing entry is then inserted. At least in some embodiments, if multiple candidate buckets are tied for being least-full, then the designated component selects the candidate bucket belonging to the least-full routing table.

In one embodiment, if the least-full bucket is nevertheless full, an existing routing entry in the least-full bucket is discarded, in order to make room for inserting the new routing entry. In some embodiments, the discarded routing entry is not reinserted into any of the hash tables. This stands in contrast to some alternative approaches for hash table management, e.g., cuckoo hashing. In other embodiments, the discarded routing entry is reinserted into one of the hash tables using the same mechanism—i.e., by generating a hash value from each hash function and identifying the least-filled bucket, into which the discarded routing entry is inserted. At least in some embodiments, each routing entry includes a flag indicating whether the respective routing entry is valid. In such embodiments, a routing entry may be discarded by setting the flag to indicate that the routing entry is invalid.

In one embodiment, the routing key has Layer-2 routing information including a virtual local area network (VLAN) tag, a logical network (LN) identifier, and a media access control (MAC) address, each of which is further described below. The VLAN tag may indicate an assigned VLAN, which may be used to segregate traffic and to allow more than one uplink. There may be multiple VLANs on a single uplink. In some embodiments, each VLAN uses only one uplink port. In other words, only one physical uplink port may be used at any given time to forward a data frame associated with a given VLAN. In other embodiments, through the use of logical networks, a VLAN may use multiple physical ports to forward traffic while also maintaining traffic separation. Further, link aggregation may be used to bundle several physical links to act as one uplink with increased bandwidth.

In one embodiment, each logical network may have a respective LN identifier and may include a respective, logically specified network portion of the distributed network switch. Further, multiple logical networks may be included within a single bridge element. As such, a logical network may provide an additional layer of traffic separation. When so configured, logical networks may allow different clients to use the same VLAN tag. The VLANs of each client may remain segregated by virtue of the different logical networks.

In some embodiments, the MAC address may be generated and assigned by an Fibre Channel Forwarder (FCF), which may be a component of a controlling bridge operatively connected to the LRI 406. A FCF, also referred to herein as an FCoE switch, is configured to facilitate connectivity between FCoE initiators and Fibre Channel fabrics. For example, an FCoE data frame sent from a first virtual machine and intended for a second virtual machine may be addressed to the FCF in accordance with the FCoE standard. The FCF may receive and readdress the FCoE data frame for forwarding to the second virtual machine. The MAC address of the FCF may have been learned by the first server 105 during a discovery phase, when the FCF establishes communications with networked devices. During the discovery phase, the second server 106 may respond to broadcast queries from the first server 105. The FCF may discover the second server based on the query responses. After the discovery phase, a login phase may be initiated. A MAC address of the second server 106 may be reassigned by the FCF. The reassigned MAC address may be used for subsequent routing and communications between the servers 105, 106. The FCF may facilitate storage of MAC addresses assigned to the servers 105, 106.

At least in some embodiments, the routing table of each switch module serves as a standalone routing table in its own right and does not operate as a cache of any global routing table. In some alternative embodiments, however, the routing tables of each switch module may have a format similar to a global routing table of the controlling bridge operatively connected to the LRI 406. In such embodiments, the routing table of a given switch module may effectively serve as a local cache for a subset of entries of the global routing table of the controlling bridge. The routing tables may have a smaller capacity than the global routing table of the controlling bridge. The routing tables may be updated with routing information learned as a result of data frames flowing through the bridge elements of the distributed network switch.

In one embodiment, the routing tables may additionally be updated with routing information from the global routing table. For example, a given bridge element may receive a data frame that includes a destination MAC address that is not found in the routing table local to the given bridge element. To obtain routing information for forwarding the data frame, the bridge element may send a query to a second bridge element configured to access the controlling bridge. The second bridge element may search the global routing table for routing information associated with the destination MAC address. If the routing information is found in the global routing table, the second bridge element may forward the routing information through the distributed network switch and to the querying bridge element. The querying bridge element may store the routing information within the routing table local to the querying bridge element. Further, added, updated, or invalidated routing information in one or more of the routing tables may also be propagated to the global routing table of the controlling bridge. For example, a bridge element may send an update message to the controlling bridge in response to learning an updated or new MAC address. The global routing table may then be updated to reflect the updated or new MAC address.

In one embodiment, each routing entry in the routing table is uniquely identified using an associated entry address. As an example, a twenty-bit entry address 510 for a routing entry 508 in the bucket 504 is shown. The entry address 510 includes a 2-bit hash table identifier 512, a 2-bit slice identifier 514, a 13-bit bucket identifier 516, and a 3-bit routing entry offset 518. As shown, the routing entry 508 is a third entry in the bucket 504 of a first slice 506 ₅ of the second hash table 502 ₂ within the routing table. Assume the bucket 504 is a second-to-last bucket in the first slice 506 ₅ of the second hash table 502 ₂. The routing entry 508 may then be uniquely identified in the routing table via an entry address of 01001111111111110010 where, starting with the most significant bit, the first two bits of 01 refer to the second hash table 502 ₂, the next two bits of 00 refer to the first slice 506 ₅ within the second hash table 502 ₂, the next thirteen bits of 1111111111110 refer to the second-to-last bucket 504 within the first slice 506 ₅, and the last three bits of 010 refer to a location of a third routing entry within the second-to-last-bucket 504.

By configuring the routing table to use twenty-bit entry addresses, the routing table may support a maximum of 2̂20 or 1024K total routing entries stored in the routing table, where each slice supports a maximum of 2̂13 or 8,192 buckets, for a total of 2̂16 or 65,536 routing entries stored in the respective slice. In some scenarios, regardless of the criteria used in selecting a hash function, depending on the exact values being hashed, the hash function may not necessarily distribute a set of routing entries perfectly uniformly within a hash table. Further, regardless of the criteria used in selecting a set of different hash functions, depending on the exact values being hashed, the set of different hash functions may not necessarily distribute a set of routing entries perfectly uniformly among a set of hash tables. In one embodiment, to reduce the frequency of collisions in the routing table when routing entries are not distributed perfectly uniformly, the routing table is configured to support a maximum storage capacity that can accommodate a count of routing entries greater than an anticipated count of routing entries by a predetermined amount. Doing so populates the routing table only to a predetermined extent when storing the anticipated count of routing entries. For example, assume the anticipated count is 2̂19 or 512K routing entries. When storing the anticipated count of routing entries, the routing table described above is populated to fifty percent capacity and can operate efficiently even during times of imperfect distribution. Those skilled in the art will recognize that the number of bits used in representing the entry address and/or any part thereof may be tailored to suit the needs of a particular case, e.g., based on a total count of routing entries desired to be supported in the routing table, etc.

In one embodiment, by dividing the routing table into multiple hash tables, each hash table implemented as an independent array supporting parallel access relative to the arrays representing other hash tables, embodiments of the invention effectively increase access bandwidth supported by the routing table. The bandwidth is effectively increased because routing entries from different hash tables may be accessed in parallel. In addition, dividing the routing table into multiple hash tables as disclosed herein allows the bandwidth to be increased even further by optionally adopting a routing entry insertion mechanism that facilitates bandwidth balancing across hash tables, e.g., the steps 730 and 740 of FIG. 7 and the steps 906 and 908 of FIG. 9. For example, the routing entries may be inserted based on a set of insertion properties associated with the routing table, the set of insertion properties including bucket capacity, free hash table capacity, and slice lookup hit frequency, each of which is further described below.

In one embodiment, by dividing the each hash table into multiple slices, where each slice is implemented as an independent array supporting parallel access relative to the arrays representing other slices, embodiments of the invention effectively increase bandwidth provided by each hash table in the routing table. The bandwidth is effectively increased because routing entries from different slices may be accessed in parallel. In addition, dividing each hash table into multiple slices as disclosed herein allows the bandwidth to be increased even further by optionally adopting a routing entry insertion mechanism. For example, routing entries may be inserted into the routing table in a prescribed manner that facilitates bandwidth balancing across slices, e.g., the step of 910 FIG. 9 and the step 1040 of FIG. 10. Doing so balances access bandwidth to the routing table by evenly distributing lookup hits across hash tables and/or slices, in whole or in part.

Additionally or alternatively, bandwidth balancing across hash tables and/or slices may also be performed by preemptively assigning, to the hash tables, hash functions based on a prescribed hash function selection mechanism. For example, hash functions may be selected based on satisfying prescribed criteria for uniformly distributing access across hash tables and/or slices, in whole or in part. For instance, the hash functions may be selected based a set of hash properties including the avalanche property, the intra-hash collision property, the inter-hash collision property, and/or the inter-slice distribution property, each of which are further described above.

In one embodiment, when dividing the routing table into multiple hash tables in conjunction with dividing each hash table into multiple slices, parallel access in two dimensions of the routing table in FIG. 5 may be provided—i.e., in the horizontal direction across the hash tables 502 and in the vertical direction across slices 506. Doing so not only further increases the effective overall bandwidth provided by the routing table but also allows two-dimensional bandwidth balancing—i.e., in the horizontal direction across the hash tables 502 and in the vertical direction across slices 506. Those skilled in the art will recognize that the tradeoff between the increased bandwidth and increased processing overhead may be tailored to suit the needs of a particular case. For example, depending on the embodiment, hashing and/or slicing may be implemented using independent arrays configured to support parallel access. Additionally, owing to the hashed and/or sliced configuration of the routing table, bandwidth balancing may optionally be applied between hash tables and/or between slices—via the routing entry insertion mechanisms and/or the hash function selection mechanisms, both of which are further described herein.

FIG. 6 is a flowchart depicting a method 600 for hash-based routing table management in a distributed network switch, according to one embodiment of the invention. In particular, the method 600 is configured to update a routing table to reflect a source address. As shown, the method 600 begins at step 610, where a first switch module of the distributed network switch receives a frame having a source address and a destination address. The first switch module includes bridge elements and a routing table. The routing table is shared among the bridge elements and includes sets of buckets. Each set of buckets is associated with a respective, distinct hash function. Each bucket is configured to store up to a predetermined number of routing entries. Further, each bucket in each set of buckets is identifiable by a bucket identifier that is distinct within the respective set of buckets. In one embodiment, each set of buckets is stored in a respective hash table, and the hash tables together form the routing table of the first switch module.

At step 620, upon determining that the routing table in the first switch module does not include any routing entry for the source address, the first switch module determines routing information for the source address. At step 630, the first switch module generates a routing entry for the source address based on the determined routing information. At step 640, the first switch module modifies or requests to modify the routing table based on the distinct hash functions, to include the routing entry generated for the source address. The step 640 is further described below in conjunction with FIG. 7. After the step 640, the method 600 terminates.

FIG. 7 is a flowchart depicting a method 700 to modify a routing table based on distinct hash functions, according to one embodiment of the invention. The method 700 corresponds to the step 640 of FIG. 6 and may be performed by a designated component of the distributed network switch. In some embodiments, the designated component is a switch module or a component thereof, such as the arbiter component of the routing table. In other embodiments, the designated component is an independent component configured to manage the routing table in response to requests received from switch modules in the distributed network switch.

As shown, the method 700 begins at step 710, where the designated component generates, using the hash function associated with each set of buckets, a respective hash value based on a routing key included within a header of the first frame. At step 720, the designated component identifies, within each set of buckets, a candidate bucket having a bucket identifier matching the hash value generated using the hash function associated with the respective set of buckets. At step 730, the designated component determines a least-full bucket among the identified candidate buckets. Depending on the embodiment, the least-full bucket may be the candidate bucket having a smallest count of (valid) routing entries. At least in some embodiments, if multiple candidate buckets are tied for having a smallest count of routing entries, then the designated component selects the candidate bucket belonging to the least-full hash table.

At step 740, the designated component inserts the generated routing entry into the determined least-full bucket. In one embodiment, inserting the generated routing entry includes, upon determining that the least-full bucket is full, discarding a routing entry in the least-full bucket to make room for the generated routing entry, without reinserting the discarded routing entry into any of the sets of buckets. In this regard, the routing entry may be selected to be discarded based on the routing entry having an age attribute greater than other routing entries. If multiple routing entries are tied in terms of having the greatest age attribute, then a routing entry may be selected from the multiple routing entries based on a predetermined priority scheme, to be discarded. An example of the predefined priority scheme is random selection. After the step 740, the method 700 terminates.

FIG. 8 is a flowchart depicting a method 800 to update a routing table to reflect a source address, according to one embodiment of the invention. As shown, the method 800 begins at step 810, where a second switch module of the distributed network switch receives the frame having the source address and the destination address. The second switch module includes bridge elements and a routing table. The routing table is shared among the bridge elements and includes sets of buckets, and each set of buckets is associated with a respective, distinct hash function.

At step 820, upon determining that the routing table in the second switch module does not include any routing entry for the source address, the second switch module determines routing information for the source address. At step 830, the second switch module generates a routing entry for the source address based on the determined routing information. At step 840, the second switch module modifies or requests to modify the routing table based on the distinct hash functions, to include the routing entry generated for the source address. The step 840 is further described above in conjunction with FIG. 7. After the step 840, the method 800 terminates.

FIG. 9 is a flowchart depicting a method 900 to insert a routing entry into the routing table, according to one embodiment of the invention. As shown, the method begins at step 902, where the designated component receives a request to insert a routing entry into the routing table, based on a routing key. As described above, the routing key includes a VLAN tag, a LN identifier, and a MAC address. At step 904, the designated component performs a lookup on each hash table of the routing table. At least in some embodiments, the designated component may additionally perform a set of operations including determining a least loaded bucket as a candidate bucket (step 906), determining a least-loaded hash table as a candidate table (step 908), and determine a slice that uses a least amount of bandwidth, as a candidate slice (step 910). A candidate bucket, candidate hash table, and candidate slice may also be referred to herein as a “candidate”. If a routing entry having an identical routing key already exists (step 912), the designated component optionally updates the routing entry responsive to the request (step 914).

In one embodiment, if no routing entry already exists (step 912), then the designated component determines whether the routing table is full or that all candidates are full (step 916). If not, the designated component optionally performs one or more operations 918 configured to balance bandwidth used between slices of each hash table, also referred to herein as bandwidth-balancing operations. To this end, the designated component first determines whether bandwidth insertion mode is active (step 922). If so, the designated component determines whether there is sufficient space for a predefined number of routing entries in the bucket of the candidate slice (step 924). If so, the designated component inserts the routing entry into the bucket of the candidate slice (step 926). Otherwise, the designated component determines whether all candidate buckets are equally filled with valid routing entries (step 928). If so, the routing entry is inserted in the least-filled hash table (step 932). Otherwise, the routing entry is inserted in the least-filled bucket (step 930).

On the other hand, if the routing table is full or all candidates are full (step 916), then the method 900 terminates subsequent to one or more additional operations 920. The additional operations 920 include replacing a duplicate routing key in the hash tables if the duplicate routing key exists (steps 934 and 936), replacing a first aged routing key from the routing tables if the aged routing key exists (steps 938 and 940), and replacing a first non-static routing entry based on a predefined pointer if the non-static routing entry exists (steps 942 and 944). In one embodiment, the predefined point is a round-robin castout pointer. Otherwise, the designed component does not insert the routing entry into the routing table but instead stores the routing entry in a predetermined location, such as a configuration registry. At least in some embodiments, one or more of the additional operations 920 are optional. Additionally or alternatively, one or more of the steps of the method 900 may be optional.

FIG. 10 is a flowchart depicting a method 1000 for sliced routing table management in the distributed network switch, according to one embodiment of the invention. In particular, the method 1000 is configured to update a sliced routing table to reflect a source address. As shown, the method 1000 begins at step 1010, where the first switch module of the distributed network switch receives a frame having a source address. The first switch module includes bridge elements and a sliced routing table. The sliced routing table is shared among the bridge elements and includes sets of buckets, and each set of buckets is associated with a respective, distinct hash function as described above. Further, each set of buckets is divided into slices, each slice having a respective property and including one or more buckets. For example, each slice may have an access frequency property that is maintained by the arbiter component of the distributed network switch, based on monitored access to the sliced routing table.

At step 1020, upon determining that the sliced routing table in the first switch module does not include any routing entry for the source address, the first switch module determines routing information for the source address. At step 1030, the first switch module generates a routing entry for the source address based on the determined routing information. At step 1040, the first switch module modifies or requests to modify the sliced routing table based on the distinct hash functions and the respective properties of the slices, to include the routing entry generated for the source address. The step 1040 is further described above in conjunction with the operations 918 of FIG. 9. After the step 1040, the method 1000 terminates. Configuring and operating the routing table as disclosed herein may increase access bandwidth provided by the routing table at least in some cases.

FIG. 11 is a data flow diagram 1100 illustrating operations for managing a cached routing table, according to one embodiment of the invention. As shown, the data flow diagram 1100 includes an arbiter component 1102 managing access to slices 506 of a routing table. As shown, the routing table includes four separate hash tables, denoted in FIG. 11 as HT0 through HT3, respectively. As described above, each hash table is associated with a distinct hash function. Further, each hash table may optionally be divided into four slices, denoted in FIG. 11 as S0 through S3, respectively. Each slice has a predetermined number of buckets, and each bucket includes eight routing entries 1130 in the example shown in FIG. 11. The routing entries 1130 are denoted in FIG. 11 as Entry0 through Entry7, respectively.

In one embodiment, the arbiter component 1102 is configured to handle routing table access requests 1114 from the bridge elements 402 of the switch module, where the bridge elements include five dVEBs in the example of FIG. 11. The bridge elements 402 may send these routing table access requests 1114 responsive to source address lookup requests and/or destination address lookup requests initiated by a predetermined entity, such as the bridge elements 402 or a frame pre-parser component of the switch module. The arbiter component 1102 may also be configured to handle insert/update requests 1122 and/or refresh requests 1128. In a particular embodiment, each request that the arbiter component is configured to handle is an embedded dynamic random access memory (eDRAM) request. In one embodiment, the insert/update requests 1122 may be generated upon detecting collisions 1126 from a hashing operation 1124 that is performed as a result of an operation selected from a set of predefined operations 1120. The set of predefined operations 1120 includes an insert/update request initiated by a parser component for received frames, receipt of a duplicate request, initiation of an aging or refresh function, and other operations without departing from the scope of the present disclosure. For example, the other operations may include software operations such as, without limitation, insert key, invalidate key, read key, read index, move key, flush, refresh sync, and initialize.

In one embodiment, each of the five dVEBs generates two lookup requests per frame, including a source address lookup request and a destination address lookup request, for a total of ten accesses to the routing table that is shared between the five dVEBs. Each of the five dVEBs includes queues 1110. In one embodiment, the queues 1110 include a request queue for source address lookups and a request queue for destination address lookups. Each lookup request may be added to and subsequently removed from the queue to perform a respective hashing operation 1104, 1106 based on the respective lookup request. In some embodiments, lookup requests may also bypass the queues, and the hashing operations 1104, 1106 are performed on the lookup requests.

In one embodiment, the hashing operations 1104, 1106 include applying one or more of the distinct hash functions of the hash tables based on the lookup requests 1112, to generate one or more routing table access requests 1114. In one embodiment, a total count of routing table access requests 1114 equal to a total count of hash tables included in the routing table is generated for each lookup request 1112. The arbiter component 1102 facilitates accessing the routing table at least in terms of read, write, and/or refresh operations and returns a result to the requesting dVEB. The result may include an identified routing entry or an indication that no routing entry was identified. In one embodiment, the dVEB may then transmit the result to a predetermined component such as a frame receive pipeline component of the switch module and/or forward the frame based on routing information contained in the result, if any.

In one embodiment, each of the five dVEBs is configured to include a respective set of caches. Depending on the embodiment, the set of caches may include one or more caches. In a particular embodiment, the set of caches includes a source address lookup cache and a destination address lookup cache. The lookup caches may be operatively connected to the source address request queue and the destination address request queue, respectively, in such a way as to precede the request queues in terms of processing, at least from the perspective of operations involved when handling incoming lookup requests 1112. At least in some embodiments, the lookup caches may also be operatively connected to a lookup interface of the respective dVEB.

In one embodiment, each lookup cache of each bridge element may store up to a predefined count of routing entries previously requested by and resolved for the respective bridge element. In one embodiment, a predetermined caching policy may be used to determine which routing entries should be stored by each lookup cache. For example, the predetermined caching policy may specify to store up to a predefined count of routing entries most recently resolved for the respective bridge element. The predetermined caching policy may be tailored by a user to suit the needs of a particular case.

In one embodiment, upon a cache miss, which is characterized by no routing entry being identified in a given cache of the set of caches 1108, corresponding to the lookup request 1112, a buffer reorder operation 1118 is optionally performed to store lookup responses for a predetermined period of time, including source address lookup responses and/or destination address lookup responses. Doing so facilitates sending the lookup responses to the requesting dVEB at the same time and/or in the same order as the corresponding lookup requests, including source address lookup requests and destination address lookup requests. More specifically, in one embodiment, the source address lookup cache corresponds to source address lookup requests and not to destination address lookup requests. Likewise, the destination address lookup cache corresponds to destination address lookup requests and not to source address lookup requests. Upon a cache hit, which is characterized by a routing being identified in a given cache of the set of caches 1108, corresponding to the lookup request 1112, processing then proceeds to the hashing operations 1104, 1106, and the buffer reorder operation 1118 is optionally performed responsive to receiving results from the arbiter component 1102. At least in some embodiments, the buffer reorder operation 1118 is performed in accordance with the predetermined caching policy described above.

In one embodiment, when processing a source address lookup request, the routing key associated with the source address lookup request is compared against the routing keys of all valid routing entries stored in the source address lookup cache. If a match is found, then the routing entry from the source address lookup cache is returned as a responsive result, without requiring the routing table to be accessed. Similarly, when processing a destination address lookup request, the routing key associated with the destination address lookup request is compared against the routing keys of all valid routing entries stored in the destination address lookup cache. If a match is found, then the routing entry from the destination address lookup cache is returned as a responsive result, without requiring the routing table to be accessed.

In a particular embodiment, each lookup cache stores up to sixteen routing entries and includes latches to facilitate comparison and insertion operations. Further, each lookup cache stores routing entries resulting from the sixteen most recent lookup requests yielding a hit in the routing table. Each routing entry newly inserted into the lookup cache overwrites an oldest routing entry in the lookup cache. In some embodiments, a pointer value is maintained to indicate a next location in the lookup cache into which to insert a next routing entry.

In one embodiment, each lookup cache of each bridge element is configured to be invalidated upon routing information of any valid routing entry being modified. Each lookup cache of each bridge element is further configured to be invalidated upon receiving an indication that a flush operation is requested by a predetermined component of the distributed network switch, such as the IOMC 140. Each lookup cache of each bridge element is still further configured to be invalidated upon a threshold duration of time elapsing since a last invalidation of the respective lookup cache. The threshold may be configured by a user to suit the needs of a particular case. In some embodiments, invalidating a lookup cache includes invalidating all routing entries stored in the lookup cache. In alternative embodiments, fewer than all routing entries stored in the lookup cache are invalidated. Configuring the lookup caches to be invalidated as disclosed herein facilitates removal of stale routing information from the lookup caches. Further, the lookup cache need not be flushed upon an aging out of each routing entry in the routing table. Further still, routing entries need not be compared prior to inserting the routing entries into the set of caches.

FIG. 12 is a flowchart depicting a method 1200 for cached routing table management in the distributed network switch, according to one embodiment of the invention. In particular, the method 1200 is configured to update a cached routing table to reflect one or more addresses. As shown, the method 1200 begins at step 1210, where the first switch module of the distributed network switch receives a first frame having a source address and a destination address. The first switch module includes bridge elements and a routing table. The routing table is shared among the bridge elements and includes sets of buckets, and each set of buckets is associated with a respective, distinct hash function as described above. Further, a first bridge element of the bridge elements has an associated set of caches. In some embodiments, each bridge element has a respective, associated set of caches. As described above, the set of caches may include one or more caches. In some embodiments, each set of caches includes a source address lookup cache and a destination address lookup cache.

At step 1220, upon determining that the routing table does not include a routing entry for the source address, the first switch module generates a routing entry for the source address, based on routing information determined for the source address. At step 1230, the first switch module modifies or requests to modify the routing table based on the hash functions, to include the routing entry generated for the source address, and the first frame is forwarded based on the routing information determined for the destination address. The step 1230 is further described above in conjunction with FIG. 7. At step 1240, upon accessing the generated routing entry in the modified routing table responsive to a subsequent lookup request for the source address, the first switch module modifies the set of caches to include the generated routing entry. At least in some embodiments, a source address routing entry is inserted into source address lookup cache only upon each source address lookup hit, and a destination address routing entry is inserted into the destination address lookup cache only upon each destination address lookup hit. After the step 1240, the method 1200 terminates.

FIG. 13 is a flowchart depicting a method 1300 for accessing a cached routing table in the distributed network switch, according to one embodiment of the invention. In particular, the method 1300 is configured to query a cached routing table. As shown, the method 1300 begins at step 1310, where the first switch module of the distributed network switch receives a second frame having an address selected from the source address of the first frame and the destination address of the first frame. At step 1320, the first switch module accesses the set of caches of the first bridge element to identify a routing entry for the address of the second frame, without accessing the routing table in the first switch module. At least in some embodiments, accessing the routing table incurs a higher performance cost than accessing the set of caches. In some embodiments, accessing the routing table may incur a performance cost that is one or more orders of magnitude greater than accessing the set of caches.

At step 1330, the first switch module determines if any routing entry is identified. If not, the first switch module then accesses the routing table in the first switch module to identify the routing entry for the address of the second frame (step 1335). If a routing entry is identified in the set of caches or in the routing table, then the first switch module then forwards the second frame based on routing information contained in the identified routing entry (step 1340). At least in some embodiments, if the set of caches and the routing table each does not contain any routing entry for the address of the second frame, the first switch module may perform, at least in part, the steps 1220, 1230, 1240 of FIG. 12, prior to forwarding the second frame based on the determined routing information. After the step 1340, the method 1300 terminates.

FIG. 14 is a flowchart depicting a method 1400 for determining to invalidate a lookup cache associated with the first bridge element, according to one embodiment of the invention. As shown, the method 1400 begins at step 1410, where the first bridge element determines if the routing information of any valid routing entry is modified. If not, the first bridge element determines if a flush operation is requested by a master controller component of the distributed network switch (step 1420). If not, the first bridge element determines if a threshold duration of time has elapsed since a last invalidation of the lookup cache by the first bridge element. Upon an affirmative determination of any of the steps 1410, 1420, and 1430, the first bridge element invalidates all routing entries in the lookup cache (step 1440). Otherwise, the method 1400 terminates without invalidating any routing entries in the lookup cache. By managing a cached routing table according to the techniques disclosed herein, the lookup caches may be transparent both to firmware and to hardware synchronization messages passed between routing tables, and frequency of accessing the routing tables may be reduced, resulting in improved performance of the distributed network switch at least in some cases.

FIG. 15 is a block diagram depicting components for sliced routing table management with replication, according to one embodiment of the invention. As described above, the routing table 404 includes the predetermined number of hash tables 502 ₁₋₄, each hash table 502 ₁₋₄ having a predetermined number of buckets, such as buckets 1502, 1504. Each bucket is configured to store up to a predetermined number of routing entries that include routing information. Further, each hash table may include the predetermined number of slices 506, each slice implemented as an independent array data structure configured to support parallel access, relative to the other slices, and each slice may be associated with one or more properties specific to the respective slice.

In some embodiments, the first switch module may frequently access a subset of entries of a given slice. Frequent access to entries of a given slice is also referred to herein as a polarized lookup pattern or polarized lookups. For example, there are instances in which all five ports of each of the five dVEBs may use the same subset of MAC addresses. Further, characteristics of the hash functions may place many of the routing entries for the MAC addresses within the same slice in one of the hash tables of the routing table. Even when the distributed network switch is configured to include sets of caches, the frequency of cache misses may nevertheless be unacceptable at least in some scenarios—such as where the count of source and destination MAC addresses used is greater than a count of routing entries supported by the caches at the front of the request queues. Thus, polarized lookups may reduce or eliminate any performance gains that hash table slicing and routing entry caching is otherwise configured to provide to the distributed network switch.

In one embodiment, to reduce or eliminate decreases in performance that are associated with polarized lookups, the distributed network switch is further configured to detect slices in which polarized lookups occur and to replicate one or more frequently accessed routing entries in the detected slices to one or more other slices in one or more other hash tables. Doing so provides improved, parallel access to the routing entries accessed by the polarized lookups, thereby improving an effective bandwidth for subsequent accesses of the routing table, to an extent that the subsequent accesses having a similar polarized lookup pattern.

As described above, in one embodiment, the arbiter component of the distributed network switch is configured to monitor and maintain one or more properties of each slice in the routing table. An example of a property of a slice is a frequency with which a hit occurs in slice responsive to a lookup request—i.e., the frequency with which a valid routing entry is identified in the slice, responsive to a source address lookup request or a destination address lookup request. The frequency may be measured over a predetermined time duration, also referred to herein as a slice bandwidth measurement period, or measurement period. In some embodiments, a counter representing a lookup hit frequency is maintained for each slice. The counter may be reset to zero for each slice upon entering a new slice bandwidth measurement period. The counter for a given slice may be incremented upon the occurrence of each lookup hit in the given slice.

In one embodiment, if the property of a given slice satisfies a predetermined replication condition—such as the measured frequency for a given slice exceeding a predetermined frequency threshold—the arbiter component may replicate, or attempt to replicate, one or more of the accessed routing entries in the slice to other slices in one or more other hash tables. For example, a routing entry in the bucket 1502 of the first hash table may be replicated to a bucket 1504 of the second hash table. To this end, the hash function associated with the second table may be applied based on the routing entry to be replicated, to determine a slice identifier, bucket identifier, and/or routing entry identifier with which to insert the routing entry. The slice identifier, bucket identifier, and/or routing entry identifier in the second hash table, which identify where a replicated copy of the routing entry is to be inserted, may not necessarily correspond to the slice identifier, bucket identifier, and/or routing entry identifier in the first hash table, which identify where the original routing entry is stored.

In one embodiment, at least a minimum count and up to a maximum count of accessed routing entries may be replicated for a given slice and instance of the measurement period—or alternatively, for a given time period overall. The predetermined time duration, the predetermined replication condition, the predetermined frequency threshold, the minimum count, and the maximum count may be tailored based on user input to suit the needs of a particular case.

In one embodiment, at the end of each slice bandwidth measurement period, the counters may be logged and compared to the predetermined frequency threshold, to determine whether to replicate one or more accessed routing entries to other slices of the hash table. In some embodiments, each routing entry includes a flag indicating whether the respective routing entry is an original routing entry or a replicated copy, also referred to herein as a replicated routing entry. In some embodiments, only original routing entries may be replicated to other slices of the hash table; replicated copies are not themselves further copied to other slices of the hash table. Further, aging may apply independently to original routing entries and replicated copies, respectively. In some embodiments, refresh synchronization messages associated with original routing entries may be sent, while no refresh synchronization messages associated with the replicated copies are sent.

In one embodiment, once replicated copies of a routing entry in the given slice is inserted into other slices of other hash tables, subsequent lookup requests for the routing entry may be resolved in parallel via accessing multiple slices of the hash table—rather than being resolved by queuing the lookup requests to access only the given slice in the hash table. As a result, the subsequent lookup requests may be resolved more efficiently at least in some cases. Further, when a request for a routing entry is resolved via accessing one of the multiple slices of the hash table, then any outstanding requests for the routing entry in the other slices are dropped, such that no further handling of the outstanding requests is performed.

FIG. 16 is a flowchart depicting a method 1600 for sliced routing table management with replication, according to one embodiment of the invention. As shown, the method 1600 begins at step 1610, where the first switch module of the distributed network switch receives a first frame having a source address and a destination address. The first switch module includes bridge elements and a routing table that is shared among the bridge elements. The routing table includes sets of buckets, and each set of buckets is associated with a respective, distinct hash function. Further, each set of buckets is divided into slices, each slice having a respective property and including one or more buckets. At step 1620, the arbiter component of the first switch module determines that a first slice of a first set of buckets of the routing table includes a routing entry for the source address and that the property of the first slice satisfies a replication condition. At step 1630, the arbiter component replicates the routing entry to at least a second set of buckets of the routing table. After the step 1630, the method 1600 terminates. Although described herein with reference to the source address, at least in some embodiments, the steps of the method 1600 may be adapted to additionally or alternatively operate with the destination address. In such embodiments, upon accessing a routing entry for a destination address in a first slice of a first set of buckets of the routing table responsive to a lookup request for the destination address, and determining that the first slice satisfies a replication condition, the arbiter component replicates the routing entry to at least a second set of buckets of the routing table.

FIG. 17 is a flowchart depicting a method 1700 for determining whether to replicate a routing entry in a distributed network switch, according to one embodiment of the invention. The method 1700 corresponds to steps 1610 through 1630 of FIG. 16. As shown, the method 1700 begins at step 1710, where the arbiter component of the distributed network switch monitors, for each slice, how many routing entries are identified in the respective slice responsive to address lookup requests over a given measurement period. At step 1720, the arbiter component determines whether the replication condition is satisfied. If so, then at step 1730, the arbiter component replicates one or more of the identified routing entries to one or more other sets of buckets, based on a predetermined insertion mechanism for replicated entries. The insertion mechanism of step 1730 is further described below in conjunction with FIG. 9. After the step 1730, the method 1700 terminates. In some embodiments, the arbiter component returns to the step 1710 to perform monitoring for a new measurement period.

Referring once again to FIG. 9, an insertion mechanism for new, original routing entries is shown and is previously described in the present disclosure. In one embodiment, the insertion mechanism may be adapted in the following manner to handle replicated routing entries. The adapted mechanism corresponds to step 1730 of FIG. 17. As shown, the adapted mechanism begins at step 902, where the arbiter component receives a request to insert a replicated routing entry into the routing table, based on a routing key. At step 904, the arbiter component performs a lookup on each hash table of the routing table. At least in some embodiments, the arbiter component may additionally perform a set of operations including determining a least loaded bucket as a candidate bucket (step 906), determining a least-loaded hash table as a candidate table (step 908), and determine a slice that uses a least amount of bandwidth, as a candidate slice (step 910), which is performed while skipping buckets, hash tables, and slices, respectively, that already contain the key. At step 912, the adapted mechanism determines whether an original routing entry having an identical routing key exists and, if so, further determines whether the routing key is replicated to all other hash tables. At least in some embodiments, the original routing entry is characterized by having a replication flag set to false. If no original routing entry having an identical routing key exists, or if the routing key is replicated to all other hash tables, the arbiter component refrains from inserting the replicated routing entry.

On the other hand, if the routing key is not replicated to all other hash tables, then the arbiter component determines whether the routing table is full or that all candidates are full (step 916). If so, the arbiter component refrains form inserting the replicated routing entry. Otherwise, the arbiter component optionally performs one or more operations 918 configured to balance bandwidth used between slices of each hash table. To this end, the arbiter component first optionally determines whether bandwidth insertion mode is active (step 922). If so, the arbiter component determines whether there is sufficient space for a predefined number of routing entries in the bucket of the candidate slice (step 924). If so, the arbiter component inserts the replicated routing entry into the bucket of the candidate slice, the routing entry having a replication flag set to true (step 926). Otherwise, the arbiter component determines whether all candidate buckets are equally filled with valid routing entries (step 928). If so, the replicated routing entry is inserted in the least-filled hash table, the routing entry having a replication flag set to true (step 932). Otherwise, the replicated routing entry is inserted in the least-filled bucket, the routing entry having a replication flag set to true (step 930).

FIG. 18 is a flowchart depicting a method 1800 for inserting a new, non-replicated routing entry based on a predetermined priority policy, according to one embodiment of the invention. In one embodiment, the predetermined priority policy may specify, when inserting a new routing entry: (i) to favor replacing an existing, invalid entry over replacing an existing, replicated entry and (ii) to favor replacing an existing, replicated entry over replacing an oldest existing entry determined based on associated routing entry age. The predetermined priority policy may be configured based on user input to suit the needs of a particular case.

As shown, the method 1800 begins at step 1810, where the arbiter component determines whether an aging function of routing table entries is set to active. If so, then at step 1815, the arbiter component inserts the new routing entry based on an aging function insertion policy, also referred to herein as an aging insertion policy and which is further described below in conjunction with FIG. 19. Otherwise, the arbiter component determines whether a flush operation is active (step 1820). If so, the arbiter component refrains from inserting the new routing entry and terminates.

On the other hand, if no flush operation is active (step 1820), then the arbiter component determines whether the set in which to insert the new routing entry contains any invalid entries (step 1830). If so, the arbiter component replaces one of the invalid entries in the set, with the new routing entry (step 1835). Otherwise, the arbiter component determines whether the set contains any existing, replicated routing entries (step 1840). If so, the arbiter component replaces one of the existing, replicated routing entries in the set, with the new routing entry to be inserted (step 1845). Otherwise, the arbiter component replaces a most aged routing entry in the set, with the new routing entry to be inserted (step 1850). After the steps 1815, 1835, 1845, or 1850, the method 1800 terminates. At least in some embodiments, static routing entries are allowed to be replicated in the routing table.

FIG. 19 is a flowchart depicting a method 1900 for applying an aging insertion policy, according to one embodiment of the invention. The method 1900 corresponds to the step 1815 of FIG. 18. In alternative embodiments, the method 1900 is only performed subsequent to performing all steps of FIG. 18. For convenience of reference, the replicated routing entry to be inserted may also be referred to herein as the instant routing entry. In some embodiments, the aging insertion policy is applied to facilitate managing the relative locations of the original routing entry, the instant routing entry, and an aging function pointer, also referred to herein as an aging pointer. As shown, the method 1900 begins at step 1910, where the arbiter component determines whether the aging pointer in a set of buckets is past an original routing entry and before a location for inserting the instant routing entry. If so, the arbiter component inserts the instant routing entry, where the instant routing entry contains an age property set by the arbiter component to be a lowest age value in a range of predefined age values (step 1915). For example, at least in some embodiments, the lowest age value is zero.

Otherwise, at step 1920, the arbiter component determines whether the aging pointer is past the location for inserting the instant routing entry and before an original routing entry that has the lowest age value. If so, the arbiter component inserts the insert routing entry, where the instant routing entry contains age property set by the arbiter to be a second lowest age value in the range of predefined age values (step 1925). For example, at least in some embodiments, the second lowest age value is one.

Otherwise, at step 1930, the arbiter component determines whether the aging pointer is past the location for inserting the instant routing entry and before an original routing entry that has the second lowest age value. If so, the arbiter component refrains from inserting the instant entry and terminates. Otherwise, at step 1940, the arbiter component inserts the instant routing entry, where the instant routing entry contains an age property set by the arbiter component to match an age value of the original routing entry. After the steps 1915, 1925, or 1940, the method 1900 terminates.

Embodiments of the invention provide techniques for hash-based routing table management in a distributed network switch. In one embodiment, a switch module receives a frame having a source address and a destination address. If the switch module does not find any routing entry for the source address in a routing table of the switch module or in a set of caches of a bridge element of the switch module, the switch module determines routing information for the source address and generates a routing entry for the source address based on the routing information. The switch module then modifies the routing table and/or the set of caches based on the hash functions, to respectively include the routing entry generated for the source address. The routing table may also be configured to include multiple slices supporting parallel routing entry lookups, and the routing table may be modified based further on properties of the slices. Further, to further increase access bandwidth provided by the routing table, routing entries may be duplicated upon satisfaction of predefined conditions disclosed herein. Accordingly, the distributed network switch may operate more efficiently at least in some cases.

While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. 

1.-7. (canceled)
 8. A computer-readable storage medium containing a program which, when executed, performs an operation for hash-based routing table management in a distributed network switch comprising a plurality of switch modules including a first switch module, the operation comprising: receiving, by the first switch module, a first frame having a source address and a destination address, wherein the first switch module comprises a plurality of bridge elements and a routing table, wherein the routing table in the first switch module is shared among the plurality of bridge elements in the first switch module and includes a plurality of sets of buckets, wherein each set of buckets is associated with a respective hash function of a plurality of hash functions, wherein each set of buckets is divided into a plurality of subgroups of buckets, each subgroup having a respective property; and upon accessing a routing entry for the source address in a first subgroup of a first set of buckets of the routing table responsive to a first subsequent lookup request for the source address, and determining that the property of the first subgroup satisfies a replication condition, replicating the accessed routing entry to at least a second set of buckets of the routing table.
 9. The computer-readable storage medium of claim 8, wherein each subgroup includes one or more buckets, wherein the routing entry is replicated to a second subgroup of the second set of buckets, wherein the first set is different from the second set, wherein the replication condition comprises the property of the first subgroup exceeding a threshold property measure, wherein the routing entry is not replicated to any set of buckets upon determining that the first subgroup does not satisfy the replication condition, wherein each routing entry includes a replication flag indicating whether the respective routing entry is a replicated copy of a corresponding routing entry; wherein the property of the first subgroup is determined over a predetermined time duration, wherein the predetermined time duration and the threshold property measure are user-configurable, wherein a second subsequent routing entry lookup request is resolved using one of the routing entry and the replicated routing entry, wherein one or more corresponding, outstanding routing entry lookup requests are dropped, wherein the routing entry and the replicated routing entry are each configured to age over time; wherein a subsequent, newly generated routing entry is inserted into the second set at an existing entry according to a predetermined priority policy favoring an existing, invalid entry over an existing, replicated entry and further favoring the existing, replicated entry over an oldest existing entry determined based on associated routing entry age, wherein the operation further comprises: upon accessing a routing entry for the destination address in a given subgroup of a given set of buckets of the routing table responsive to a lookup request for the destination address, and determining that the property of the given subgroup satisfies a replication condition, replicating the accessed routing entry to at least a set of buckets of the routing table, different from the given set of buckets; and upon determining that the routing table does not include a routing entry for the source address, generating a routing entry for the source address, based on routing information determined for the source address, without replicating the routing entry to any set of buckets of the routing table, wherein the routing table in the first switch module is modified, based on the plurality of hash functions and the respective properties of the plurality of subgroups, to include the routing entry generated for the source address.
 10. The computer-readable storage medium of claim 9, wherein at least a first bridge element of the plurality of bridge elements has an associated set of caches, wherein the set of caches is modified upon accessing the generated routing entry in the modified routing table responsive to the first subsequent lookup request for the source address; wherein each bridge element of the plurality of bridge elements has a respective, associated set of caches, wherein each set of caches includes a source address lookup cache and a destination address lookup cache, wherein each lookup cache of each bridge element stores up to a predefined count of routing entries previously requested by and resolved for the respective bridge element, and wherein the operation further comprises: forwarding the first frame based on routing information determined for the destination address; receiving, by the first switch module, a second frame having an address selected from the source address and the destination address of the first frame; accessing the set of caches of the first bridge element to identify the routing entry for the address of the second frame, without accessing the routing table in the first switch module, wherein accessing the routing table incurs a higher performance cost than accessing the set of caches; and forwarding the second frame based on the routing entry included in the set of caches of the first bridge element.
 11. The computer-readable storage medium of claim 10, wherein each lookup cache of the first bridge element is configured to be invalidated upon each of: (i) routing information of any valid routing entry is modified; (ii) receiving an indication that a flush operation is requested by a master controller component of the distributed network switch; and (iii) a threshold duration of time has elapsed since a last invalidation of the respective lookup cache; wherein the lookup cache is not flushed upon an aging out of each routing entry in the routing table, wherein routing entries to be inserted into the set of caches need not be compared prior to insertion; wherein access to the plurality of subgroups of each set of buckets is managed by an arbiter component of the routing table, wherein each subgroup of each set of buckets is identified via a predefined subset of bits of hash function output, wherein access to each subgroup of each set of buckets is monitored by the arbiter component, wherein the respective properties of the plurality of subgroups are updated based on the monitored access.
 12. The computer-readable storage medium of claim 11, wherein each hash function is distinct, wherein each bucket stores a plurality of routing entries, wherein each set of buckets is stored in a respective hash table of the routing table in the first switch module, wherein the generated routing entry further includes a routing key included within a header of the first frame, wherein the routing key includes Layer-2 routing information comprising a virtual local area network (VLAN) tag, a logical network (LN) identifier, and a media access control (MAC) address; wherein the routing table is configured to provide parallel access to each subgroup of each set of buckets, based on the respective properties of the plurality of subgroups, wherein each subgroup includes an array data structure, wherein each hash function is selected based on: (i) an avalanche property of the respective hash function; (ii) an intra-hash collision property of the respective hash function; (iii) an inter-hash collision property of the respective hash function; and (iv) an inter-subgroup distribution property of the respective hash function.
 13. The computer-readable storage medium of claim 12, wherein the respective property of each subgroup of the plurality of subgroups comprises an access frequency property of the respective subgroup, wherein each bucket in each set of buckets is identifiable by a bucket identifier that is distinct within the respective set of buckets, and wherein modifying the routing table comprises: generating, using the hash function associated with each set of buckets, a respective hash value based on the routing key included within the header of the first frame; identifying, within each set of buckets, a candidate bucket having a bucket identifier matching the hash value generated using the hash function associated with the respective set of buckets; upon determining that an access frequency mode is enabled, determining a bucket among the identified candidate buckets, belonging to a least-frequently accessed subgroup; upon determining that the access frequency mode is disabled, determining one of: (i) a least-full bucket among the identified candidate buckets and (ii) a least-full bucket among the identified candidate buckets and belonging to a least-full set of buckets of the plurality of sets of buckets, upon determining a presence of multiple least-full buckets; and inserting the generated routing entry into the determined bucket.
 14. The computer-readable storage medium of claim 13, wherein the least-full bucket is selected from one of: (i) the candidate bucket having a smallest count of valid routing entries and (ii) the candidate bucket having a smallest count of valid routing entries and belonging to the set of buckets having a smallest count of valid routing entries; wherein inserting the generated routing entry into the determined bucket comprises: upon determining that the least-full bucket is full, discarding a routing entry in the least-full bucket to make room for the generated routing entry, without reinserting the discarded routing entry into any of the plurality of sets of buckets; wherein discarding the routing entry comprises: upon determining that a routing entry is more aged than other routing entries in the same set of buckets as the routing entry, selecting the determined routing entry to be discarded; and upon determining that a plurality of routing entries is more aged than other routing entries in the same set of buckets as the plurality of routing entries, randomly selecting a routing entry from the determined plurality of routing entries, to be discarded; wherein the plurality of switch modules further includes a second switch module comprising a plurality of bridge elements and a routing table, and wherein the operation further comprises: receiving the first frame by the second switch module; upon determining that the routing table in the second switch module does not include any routing entry for the source address, determining routing information for the source address; generating a routing entry for the source address, based on the routing information determined for the source address; and modifying the routing table in the second switch module based on the plurality of hash functions, to include the routing entry generated for the source address.
 15. A system for hash-based routing table management in a distributed network switch comprising a plurality of switch modules including a first switch module, the system comprising: one or more computer processors; a memory containing a program which, when executed by the one or more computer processors, is configured to perform an operation comprising: receiving, by the first switch module, a first frame having a source address and a destination address, wherein the first switch module comprises a plurality of bridge elements and a routing table, wherein the routing table in the first switch module is shared among the plurality of bridge elements in the first switch module and includes a plurality of sets of buckets, wherein each set of buckets is associated with a respective hash function of a plurality of hash functions, wherein each set of buckets is divided into a plurality of subgroups of buckets, each subgroup having a respective property; and upon accessing a routing entry for the source address in a first subgroup of a first set of buckets of the routing table responsive to a first subsequent lookup request for the source address, and determining that the property of the first subgroup satisfies a replication condition, replicating the accessed routing entry to at least a second set of buckets of the routing table.
 16. The system of claim 15, wherein each subgroup includes one or more buckets, wherein the routing entry is replicated to a second subgroup of the second set of buckets, wherein the first set is different from the second set, wherein the replication condition comprises the property of the first subgroup exceeding a threshold property measure, wherein the routing entry is not replicated to any set of buckets upon determining that the first subgroup does not satisfy the replication condition, wherein each routing entry includes a replication flag indicating whether the respective routing entry is a replicated copy of a corresponding routing entry; wherein the property of the first subgroup is determined over a predetermined time duration, wherein the predetermined time duration and the threshold property measure are user-configurable, wherein a second subsequent routing entry lookup request is resolved using one of the routing entry and the replicated routing entry, wherein one or more corresponding, outstanding routing entry lookup requests are dropped, wherein the routing entry and the replicated routing entry are each configured to age over time; wherein a subsequent, newly generated routing entry is inserted into the second set at an existing entry according to a predetermined priority policy favoring an existing, invalid entry over an existing, replicated entry and further favoring the existing, replicated entry over an oldest existing entry determined based on associated routing entry age, wherein the operation further comprises: upon accessing a routing entry for the destination address in a given subgroup of a given set of buckets of the routing table responsive to a lookup request for the destination address, and determining that the property of the given subgroup satisfies a replication condition, replicating the accessed routing entry to at least a set of buckets of the routing table, different from the given set of buckets; and upon determining that the routing table does not include a routing entry for the source address, generating a routing entry for the source address, based on routing information determined for the source address, without replicating the routing entry to any set of buckets of the routing table, wherein the routing table in the first switch module is modified, based on the plurality of hash functions and the respective properties of the plurality of subgroups, to include the routing entry generated for the source address.
 17. The system of claim 16, wherein at least a first bridge element of the plurality of bridge elements has an associated set of caches, wherein the set of caches is modified upon accessing the generated routing entry in the modified routing table responsive to the first subsequent lookup request for the source address; wherein each bridge element of the plurality of bridge elements has a respective, associated set of caches, wherein each set of caches includes a source address lookup cache and a destination address lookup cache, wherein each lookup cache of each bridge element stores up to a predefined count of routing entries previously requested by and resolved for the respective bridge element, and wherein the operation further comprises: forwarding the first frame based on routing information determined for the destination address; receiving, by the first switch module, a second frame having an address selected from the source address and the destination address of the first frame; accessing the set of caches of the first bridge element to identify the routing entry for the address of the second frame, without accessing the routing table in the first switch module, wherein accessing the routing table incurs a higher performance cost than accessing the set of caches; and forwarding the second frame based on the routing entry included in the set of caches of the first bridge element.
 18. The system of claim 17, wherein each lookup cache of the first bridge element is configured to be invalidated upon each of: (i) routing information of any valid routing entry is modified; (ii) receiving an indication that a flush operation is requested by a master controller component of the distributed network switch; and (iii) a threshold duration of time has elapsed since a last invalidation of the respective lookup cache; wherein the lookup cache is not flushed upon an aging out of each routing entry in the routing table, wherein routing entries to be inserted into the set of caches need not be compared prior to insertion; wherein access to the plurality of subgroups of each set of buckets is managed by an arbiter component of the routing table, wherein each subgroup of each set of buckets is identified via a predefined subset of bits of hash function output, wherein access to each subgroup of each set of buckets is monitored by the arbiter component, wherein the respective properties of the plurality of subgroups are updated based on the monitored access.
 19. The system of claim 18, wherein each hash function is distinct, wherein each bucket stores a plurality of routing entries, wherein each set of buckets is stored in a respective hash table of the routing table in the first switch module, wherein the generated routing entry further includes a routing key included within a header of the first frame, wherein the routing key includes Layer-2 routing information comprising a virtual local area network (VLAN) tag, a logical network (LN) identifier, and a media access control (MAC) address; wherein the routing table is configured to provide parallel access to each subgroup of each set of buckets, based on the respective properties of the plurality of subgroups, wherein each subgroup includes an array data structure, wherein each hash function is selected based on: (i) an avalanche property of the respective hash function; (ii) an intra-hash collision property of the respective hash function; (iii) an inter-hash collision property of the respective hash function; and (iv) an inter-subgroup distribution property of the respective hash function.
 20. The system of claim 19, wherein the respective property of each subgroup of the plurality of subgroups comprises an access frequency property of the respective subgroup, wherein each bucket in each set of buckets is identifiable by a bucket identifier that is distinct within the respective set of buckets, and wherein modifying the routing table comprises: generating, using the hash function associated with each set of buckets, a respective hash value based on the routing key included within the header of the first frame; identifying, within each set of buckets, a candidate bucket having a bucket identifier matching the hash value generated using the hash function associated with the respective set of buckets; upon determining that an access frequency mode is enabled, determining a bucket among the identified candidate buckets, belonging to a least-frequently accessed subgroup; upon determining that the access frequency mode is disabled, determining one of: (i) a least-full bucket among the identified candidate buckets and (ii) a least-full bucket among the identified candidate buckets and belonging to a least-full set of buckets of the plurality of sets of buckets, upon determining a presence of multiple least-full buckets; and inserting the generated routing entry into the determined bucket. 